Given a positive integer, return its corresponding column title as appear in an Excel sheet.

For example:

1
2
3
4
5
6
7
1 -> A
2 -> B
3 -> C
...
26 -> Z
27 -> AA
28 -> AB
1
2
3
4
5
6
7
8
9
10
11
12
public class Solution {
    public String convertToTitle(int n) {
        if (n <= 0) return null;
        StringBuilder res = new StringBuilder();
        while (n > 0) {
            n--;
            res.append((char)('A' + n % 26));
            n /= 26;
        }
        return res.reverse().toString();
    }
}